The fabrication of various solid state devices requires the use of planar substrates, or semiconductor wafers, on which integrated circuits are fabricated. The final number, or yield, of functional integrated circuits on a wafer at the end of the IC fabrication process is of utmost importance to semiconductor manufacturers, and increasing the yield of circuits on the wafer is the main goal of semiconductor fabrication. After packaging, the circuits on the wafers are tested, wherein non-functional dies are marked using an inking process and the functional dies on the wafer are separated and sold. IC fabricators increase the yield of dies on a wafer by exploiting economies of scale. Over 1000 dies may be formed on a single wafer which measures from six to twelve inches in diameter.
Various processing steps are used to fabricate integrated circuits on a semiconductor wafer. These steps include deposition of a conducting layer on the silicon wafer substrate; formation of a photoresist or other mask such as titanium oxide or silicon oxide, in the form of the desired metal interconnection pattern, using standard lithographic or photolithographic techniques; subjecting the wafer substrate to a dry etching process to remove the conducting layer from the areas not covered by the mask, thereby etching the conducting layer in the form of the masked pattern on the substrate; removing or stripping the mask layer from the substrate typically using reactive plasma and chlorine gas, thereby exposing the top surface of the conductive interconnect layer; and cooling and drying the wafer substrate by applying water and nitrogen gas to the wafer substrate.
Photoresist materials are coated onto the surface of a wafer by dispensing a photoresist fluid typically on the center of the wafer as the wafer rotates at high speeds within a stationary bowl or coater cup. The coater cup catches excess fluids and particles ejected from the rotating wafer during application of the photoresist. The photoresist fluid dispensed onto the center of the wafer is spread outwardly toward the edges of the wafer by surface tension generated by the centrifugal force of the rotating wafer. This facilitates uniform application of the liquid photoresist on the entire surface of the wafer.
Spin coating of photoresist on wafers is carried out in an automated track system using wafer handling equipment which transport the wafers between the various photolithography operation stations, such as vapor prime resist spin coat, develop, baking and chilling stations. Robotic handling of the wafers minimizes particle generation and wafer damage. Automated wafer tracks enable various processing operations to be carried out simultaneously. Two types of automated track systems widely used in the industry are the TEL (Tokyo Electron Limited) track and the SVG (Silicon Valley Group) track.
The numerous processing steps outlined above are used to cumulatively apply multiple electrically conductive and insulative layers on the wafer and pattern the layers to form the circuits. The final yield of functional circuits on the wafer depends on proper application of each layer during the process steps. Proper application of those layers depends, in turn, on coating the material in a uniform spread over the surface of the wafer in an economical and efficient manner.
During the photolithography step of semiconductor production, light energy is applied through a reticle mask onto the photoresist material previously deposited on the wafer to define circuit patterns which will be etched in a subsequent processing step to define the circuits on the wafer. Because these circuit patterns on the photoresist represent a two-dimensional configuration of the circuit to be fabricated on the wafer, minimization of particle generation and uniform application of the photoresist material to the wafer are very important. By minimizing or eliminating particle generation during photoresist application, the resolution of the circuit patterns, as well as circuit pattern density, is increased.
A reticle is a transparent plate patterned with a circuit image to be formed in the photoresist coating on the wafer. A reticle contains the circuit pattern image for only a few of the die on a wafer, such as four die, for example, and thus, must be stepped and repeated across the entire surface of the wafer. In contrast, a photomask, or mask, includes the circuit pattern image for all of the die on a wafer and requires only one exposure to transfer the circuit pattern image for all of the dies to the wafer. Reticles are used for step-and-repeat steppers and step-and-scan systems found in wafer fabrication.
Reticles must remain meticulously clean for the creation of perfect images during its many exposures to pattern a circuit configuration on a substrate. The reticle may be easily damaged such as by dropping of the reticle, the formation of scratches on the reticle surface, electrostatic discharge (ESD), and particles. ESD can cause discharge of a small current through the chromium lines on the surface of the reticle, melting a circuit line and destroying the circuit pattern.
Reticles are transferred among various stations in a semiconductor fabrication facility in reticle pods, such as SMIF (standard mechanical interface) pods. SMIF pods are generally characterized by a pod door which mates with a pod shell to provide a sealed environment in which the reticles may be stored and transferred. In order to transfer reticles between a SMIF pod and a process tool in a fab, the pod is typically loaded either manually or automatically on a load port on the process tool. Once the pod is positioned on the load port, mechanisms in the port door unlatch the pod door from the pod shell such that the reticle may be transferred from within the pod into the process tool. Another mode of reticle transfer includes the use of a wheeled cart or vehicle which includes a frame and multiple shelves on which are supported the reticles.
Within a cleanroom environment, reticles are typically hand-carried from a stocker to a step-and-scan system which is used to transfer the circuit pattern image of the reticle to the wafer substrate. Typically, the step-and-scan system can only hold two reticles at a time, whereas a succession of multiple reticles may be used in the step-and-scan operations throughout a single day. Thus, many of the reticles must be placed on a reticle vehicle or other support to await the step-and-scan procedure.
To minimize damage to the reticles by electrostatic discharge (ESD) as the reticles are hand-carried or transported from the reticle stocker to the step-and-scan system, multiple ESD eliminators are provided in the cleanroom, typically beneath the cleanroom ceiling. As is well known, such ESD eleminators (also known as static eliminators or electrostatic eliminators) typically include a pair of discharge electrodes across which a high-voltage A.C. current is applied to generate ions of each polarity. Air is blown downwardly into the cleanroom environment through vents in the ceiling, and this air carries the ions generated by the ESD eliminators to the surfaces of the reticles and prevents the buildup of electrostatic charges which may otherwise discharge and damage the reticles.
One of the problems inherent in the conventional use of multiple ESD eliminators mounted beneath the ceiling of the cleanroom is that many areas in the cleanroom lack sufficient downflow of air to facilitate sufficient transfer of the neutralizing ions from the ESD eliminators to the surfaces of the reticles. This increases the likelihood of ESD-induced damage to the reticles as they are carried or transported from the reticle stocker to the step-and-scan system and as they await their turn for the step-and-scan procedure. Accordingly, a novel reticle vehicle which is equipped with an electrostatic charge eliminator or eliminators is needed for the transport and/or temporary storage of masks or reticles in a cleanroom environment.
An object of the present invention is to provide a novel vehicle which is suitable for applications including but not limited to the transport and/or storage of masks or reticles.
Another object of the present invention is to provide a vehicle which is equipped with at least one ESD eliminator and is capable of transporting and/or temporarily storing a variety of objects while preventing electrostatic discharges on the objects.
Still another object of the present invention is to provide a novel vehicle which includes an electrostatic charge (ESC) eliminator or eliminators for eliminating electrostatic charges on objects transported and/or stored on the vehicle.
Yet another object of the present invention is to provide a novel vehicle with an electrostatic charge eliminator or eliminators which are capable of preventing electrostatic discharge damage to sensitive electronic circuits provided on devices transported and/or stored on the vehicle.
A still further object of the present invention is to provide a novel vehicle which may include a vehicle frame for supporting an object or objects and at least one electrostatic charge eliminator provided on the frame for neutralizing electrostatic charges and preventing electrostatic discharges on the object or objects.
Yet another object of the present invention is to provide a novel vehicle with electrostatic charge eliminators, which vehicle may be used in a variety of applications including but not limited to transport and/or storage of reticles or masks from a reticle or mask stocker and a stepper or scanning system for the formation of a circuit pattern from the reticle or mask onto a semiconductor wafer substrate.